1. Field of the Invention
The invention is directed to the field of sensor systems and more particularly to a sensor system that can check its operability and signal appropriately such operability.
2. Description of the Prior Art
In most prior art sensor systems the absence of a signal from the sensor is interpreted as the absence of a return signal and therefore, the absence of an object within the scan area of the sensor system. The sensor system cannot distinguish such object absence from sensor inoperability and permits the incorrect interpretation of the condition. Some sensor testing is performed by passing a separate current through the transducer leads to test for an open circuit condition but cannot test for functioning of the sensor system.
In U.S. Pat. No. 3,932,870 issued Jan. 13, 1976 an FM test signal is transmitted along with the detection signals. The return of this signal to a separate sub-Doppler signal processor can result in an alarm condition being produced. A returned test signal below a predetermined signal level will indicate that the system is not operating properly.
Green, U.S. Pat. No. 3,820,114 issued June 25, 1974 uses a test signal transmitted by the system transmitter to the system's receiver and to a distant receiver. Failure to produce the proper logical signal output shows that the system is not functioning properly.
In U.S. Pat. No. 4,382,291 issued May 3, 1983 to Nakanchi, a pattern of reflected signals is statistically compared with a reference pattern of signals prepared in advance and stored and produces an alarm when these patterns are different. Similarly, in Fullen et al, U.S. Pat. No. 4,197,537, issued Apr. 8, 1980, there is stored a reflective pattern of the area in which the system is operating and the device compares such norm or ambient signal pattern periodically with future signals from the same area. U.S. Pat. No. 4,114,146 to Inow et al. issued Sept. 12, 1978 makes use of a resultant signal from a fixed object such as a wall F and the return signal from a moving object M to produce a resultant signal of a combined phase shift. This can be compared to the phase shift of a signal returned from wall F above. No provision is made for treating a signal from object M above, but such a condition should show an error condition. The same would be true of the patent to Charlot, Jr., U.S. Pat. No. 3,731,307 issued May 1, 1973.